The poorly understood fungus Pneumocystis jirovecii is an important cause of lethal pneumonia (PCP) in immunocompromised humans, especially those with AIDS. Research approaches and clinical management of P. jirovecii pneumonia have been significantly hindered by the lack of a culture system. Attempts to develop a continuous in vitro system using standard methods have failed. We will employ a new approach, metatranscriptomics, recently used to successfully direct the supplementation of medium for a previously uncultivable bacterium. This approach will be used for the first time on an uncultivable eukaryotic pathogen. The reduced cost for next generation sequencing (NGS) of mRNA and genomic DNA has revolutionized many aspects of research. We will use NGS of mRNA to identify enzymes and transporters that are highly expressed by growing Pneumocystis populations. The substrates and targets of these proteins will be tested in an established short term in vitro culture system in an iterative fashion until an optimized culture is identified that supports the continuous propagation of Pneumocystis spp. Our objectives are to: 1) Use RNA-seq to obtain the metatranscriptome of infected rat lungs, which includes P. carinii, rat lung, and accompanying microbiota, to identify metabolic gene signatures associated with P. carinii growth and decline; 2) Use approaches of systems biology and structural bioinformatics to determine genes, key players in Pneumocystis proliferation, their possible ligands, substrates and products, to select candidate supplements for testing in vitro; 3) Evaluate modified culture media in a short term in vitro culture assay using a high throughput iterative strategy to identify essential nutrients and their concentrations that support the continuous growth of P. carinii; 4) Validate the in vitro culture using P. murina (mouse derived) and P. jirovecii (human derived) to develop a universal supplementary regime for Pneumocystis growth. Success of this project will fundamentally change the clinical diagnosis of PCP by providing a test for viable Pneumocystis and will also propel clinical and basic research forward by allowing application of powerful molecular genetic tools such as transformation and site directed mutation; facilitate drug discovery; and allow testing, tracking and investigation of drug resistance